Low sulfur coal additive for improved furnace operation

ABSTRACT

The present invention is directed to additives for coal-fired furnaces, particularly furnaces using a layer of slag to capture coal particles for combustion. The additive(s) include iron, mineralizer(s), handling aid(s), flow aid(s), and/or abrasive material(s). The iron and mineralizers can lower the melting temperature of ash in low-iron, high alkali coals, leading to improved furnace performance.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefits of U.S. ProvisionalApplication Serial No. 60/213,915, filed Jun. 26, 2000, and entitled“Low-Cost Technology to Improve Operation of Cyclone Furnaces FiringLow-Sulfur Western Coals”, which is incorporated herein by reference inits entirety.

FIELD OF THE INVENTION

[0002] The present invention relates generally to coal additives forfurnaces and specifically to coal additives for slag-type furnaces.

BACKGROUND OF THE INVENTION

[0003] Coal is widely recognized as an inexpensive energy source forutilities. Coal-fired furnaces are used to generate steam for powerproduction and industrial processes. Coal-fired furnaces have manydifferent configurations and typically include a plurality ofcombustors. In one furnace configuration, a slag layer forms on asurface of the burner and captures the coal particles for combustion.Such a furnace will be hereafter referred to as a “slag type furnace.”

[0004] An example of a combustor 100 for a slag-type furnace is depictedin FIG. 1. The depicted combustor design is used in a cyclone furnace ofthe type manufactured by Babcock and Wilcox. Cyclone furnaces operate bymaintaining a sticky or viscous layer of liquid (melted) ash (or slag)(not shown) on the inside cylindrical walls 104 of the cyclonecombustion chamber 108. Coal is finely crushed (e.g., to minus ¼ inchtop size), entrained in an airstream, and blown into the combustor end112 of the cyclone combustor or combustor 100 through coal inlet 116.Combustion air (shown as primary air 120, secondary air 124, andtertiary air 128) is injected into the combustion chamber 108 to aide incombustion of the coal. The whirling motion of the combustion air (hencethe name “cyclone”) in the chamber 108 propels the coal forward towardthe furnace walls 104 where the coal is trapped and burns in a layer ofslag (not shown) coating the walls. The re-entrant throat 140 (whichrestricts escape of the slag from the chamber 108 via slag tap opening144) ensures that the coal particles have a sufficient residence time inthe chamber 108 for complete combustion. The slag and other combustionproducts exit the chamber 108 through the slag tap opening 144 at theopposite end from where the coal was introduced. The molten slag (notshown) removed from the chamber 108 flows to a hole (not shown) in thebottom of the boiler where the slag is water-quenched and recovered as asaleable byproduct. The ash composition is important to prevent the slagfrom freezing in the hole and causing pluggage. To melt ash into slag atnormal combustion temperatures (e.g., from about 2600 to about 3000°F.), slag-type furnaces, such as cyclones, are designed to burn coalswhose ash contains high amounts of iron and low amounts of alkali andalkaline earth metals (as can be seen from FIG. 2). Iron both reducesthe melting temperature of the ash and increases the slag viscosity atthese temperatures due to the presence of iron aluminosilicate crystalsin the melt.

[0005] High sulfur content in coal, particularly coals from the easternUnited States, has allegedly caused significant environmental damage dueto the formation of sulfur dioxide gas. As a result, utilities areturning to low sulfur western coals, particularly coals from the PowderRiver Basin, as a primary feed material. As used herein, “high sulfurcoals” refer to coals having a total sulfur content of at least about1.5 wt. % (dry basis of the coal) while “low sulfur coals” refer tocoals having a total sulfur content of less than about 1.5 wt. % (drybasis of the coal) and “high iron coals” refer to coals having a totaliron content of at least about 10 wt. % (dry basis of the ash) while“low iron coals” refer to coals having a total iron content of less thanabout 10 wt. % (dry basis of the ash). As will be appreciated, iron andsulfur are typically present in coal in the form of ferrous or ferriccarbonites and/or sulfides, such as iron pyrite.

[0006] The transition from high sulfur (and high iron) to low sulfur(and low iron) coals has created many problems for slag-type coalfurnaces such as cyclone furnaces. When low-sulfur western coals, withlow iron and high (i.e., at least about 20 wt. % (dry basis of the ash))alkali (e.g., calcium) contents, are fired in these boilers, theviscosity of the slag is too low, causing less retained bottom ash (or ahigher amount of entrained coal and ash particulates in the offgas fromcombustion), degraded performance of particulate collectors (due to theincreased particulate load) and therefore a higher incidence of stackopacity violations and increased fuel and maintenance costs, lessreliable slag tapping, the occurrence of flames in the main furnace,high furnace exit temperatures (or sprays), and increased convectivepass fouling. As shown in FIG. 3, in the operating range noted abovehigh sulfur coals (denoted as Illinois coal) form slag having a moderateto high viscosity and therefore produce a relatively thick slag layer onthe surface of the furnace while low sulfur coals (denoted as PRB coals)form a slag having a very low viscosity and therefore produce thin, lowviscosity slag layers. As a result, utilities using slag-type furnaces,such as cyclone furnaces, have, through switching feed materials,realized lower sulfur dioxide emissions but at the same time haveproduced a host of new operational problems.

[0007] Techniques that have been employed to provide improved slagcharacteristics for high sulfur eastern coals have proven largelyineffective for low sulfur coals. For example, limestone has been usedby utilities as a high sulfur coal additive to adjust the slag viscosityto the desired range for the furnace operating temperature. The calciumin the limestone is widely believed to be the primary reason for theimproved performance. Low sulfur western coals, in contrast, alreadyhave relatively high calcium contents and therefore experience little,if any, viscosity adjustment when limestone is added to the coal feed tothe furnace.

[0008] Another possible solution is the addition of iron pellets (whichtypically include at least predominantly nonoxidized iron) to thefurnace to assist in slag formation and coal combustion. Iron oxidefluxes high-silica glass, while reduced forms of iron (FeO or Fe-metal)flux calcium-rich glass. In the presence of burning coal particles, ironexists primary in reduced form. The use of iron has been recommended tosolve slag-tapping problems in cyclone furnaces by adding commerciallyavailable iron pellets, which are very expensive. The pellets have afurther disadvantage of forming pools of reduced iron that can be verycorrosive to metal or refractory surfaces exposed to the iron and/or ofbeing an ineffective fluxing agent. Therefore, iron fluxes have failedto achieve long term acceptance in the utility industry.

[0009] Another possible solution is to blend high iron coals with thewestern coals to increase the iron content of the coal feed. Blendedcoals are far from a perfect solution. High iron coals (or “kicker”coals) are often much more expensive coals than western coals. High ironcoals also have high sulfur levels because the predominant form of ironin such coals is iron sulfide (or iron pyrite). Blended coals sufferfrom increased operating costs and increased sulfur dioxide emissions,which can in certain cases exceed applicable regulations.

[0010] Another possible solution is to grind the coal going into thecyclone furnace much finer and supply additional air to increase thepercentage of combustion that occurs for coal particles in flight. Thisoption requires expensive modifications or replacement of grindingequipment and is counter to the original design and intent of thecyclone furnace. The technique further decreases boiler efficiency andincreases the auxiliary power required to operate the boiler. The use offine grinding has thus proven to be an inadequate solution to theproblem in most cases.

SUMMARY OF THE INVENTION

[0011] The various methods and compositions of the present invention canprovide a fluxing agent or additive that can be contacted with the coalfeed to or in a combustion chamber of a furnace to produce a slag layerhaving one or more desirable characteristics, such as viscosity andthickness. The methods and compositions are particularly effective for acyclone furnace of the type illustrated in FIG. 1.

[0012] In one embodiment, a method is provided for combusting coal thatincludes the steps of:

[0013] (a) providing a coal-containing feed material to a coalcombustion chamber;

[0014] (b) contacting the feed material with an iron-containingadditive; and

[0015] (c) melting at least a portion of the coal-containing feedmaterial and iron-containing additive to form a slag layer on at least aportion of a surface of the combustion chamber, whereby coal in thecoal-containing feed material is captured by the slag layer andcombusted. As noted below, the additive permits slag-type furnaces toburn low iron, high alkali, and low sulfur coals by enhancing theslagging characteristics of the ash.

[0016] The coal-containing feed material has coal as the primarycomponent. As used herein, “coal” refers to macromolecular networkcomprised of groups of polynuclear aromatic rings, to which are attachedsubordinate rings connected by oxygen, sulfur and aliphatic bridges.Coal comes in various grades including peat, lignite, sub-bituminouscoal and bituminous coal. In one process configuration, the coalincludes less than about 1.5 wt. % (dry basis of the coal) sulfur whilethe coal ash contains less than about 10 wt. % (dry basis of the ash)iron as Fe₂O₃, and at least about 15 wt. % calcium as CaO (dry basis ofthe ash). The material is preferably in the form of a free flowingparticulate having a P₉₀ size of no more than about 0.25 inch.

[0017] The coal combustion chamber is part of a coal-fired furnace. Thefurnace can be of any configuration, with a slag-type furnace beingpreferred and a cyclone furnace being even more preferred.

[0018] The iron-containing additive can be in any form and anycomposition so long as iron is present in sufficient amounts to fluxeffectively the feed material. The iron can be present in any form(s)that fluxes under the conditions of the furnace, including in the formsof ferrous or ferric oxides and sulfides. In one formulation, iron ispresent in the form of both ferric and ferrous iron, with ferric andferrous iron oxides being preferred. Preferably, the ratio of ferric (orhigher valence) iron to ferrous (or lower valence) iron is less than 2:1and more preferably ranges from about 0.1:1 to about 1.95:1, or morepreferably at least about 33.5% of the iron in the additive is in theform of ferrous (or lower valence) iron and no more than about 66.5% ofthe iron in the additive is in the form of ferric (or higher valence)iron. In a particularly preferred formulation, at least about 10% of theiron in the additive is in the form of wustite. “Wustite” refers to theoxide of iron of low valence which exist over a wide range ofcompositions (e.g., that may include the stoichiometric composition FeO)as compared to “magnetite” which refers to the oxide of iron ofintermediate or high valence which has a stoichiometric composition ofFe₂O₃ (or FeO.Fe₂O₃). It has been discovered that the additive isparticularly effective when wustite is present in the additive. Whilenot wishing to be bound by any theory, it is believed that the presenceof iron of low valence levels (e.g., having a valence of 2 or less) inoxide form may be the reason for the surprising and unexpectedeffectiveness of this additive composition.

[0019] While not wishing to be bound by any theory, it is believed thatthe presence of iron in the calcium aluminosilicate slags of westerncoals causes a decrease in the melting temperature of the ash andcrystal formation in the melt when a critical temperature (T_(cv)) isreached. These crystals change the flow characteristics of the slagcausing the slag to thicken before the slag can flow. This phenomenon isknown as “yield stress” and is familiar to those skilled in the art ofnon-Newtonian flow. Thicker slag allows the slag to capture and holdmore coal particles. Therefore, fewer coal particles escape thecombustor without being burned.

[0020] In a preferred process configuration, the additive is in the formof a free-flowing particulate having a P₉₀ size of no more than about300 microns (0.01 inch) and includes at least about 50 wt. % iron.Compared to iron pellets, the relatively small particle size of theadditive reduces significantly the likelihood of the formation of poolsof reduced iron that can be very corrosive to metal or refractorysurfaces exposed to the iron. It is believed that the reason for poolingand poor fluxing has been the relatively large sizes of iron pellets(typically the P₉₀ size of the pellets is at least about 0.25 inch (6350microns)) in view of the short residence times of the pellets in thecombustion chamber. Such pellets take longer to heat and therefore meltand act as a flux. This can cause the pellets to pass or tumble throughthe chamber before melting has fully occurred. The increase surface areaof the additive further aids in more effective fluxing as more additivereaction surface is provided.

[0021] Preferably, the additive further includes a mineralizer, such aszinc oxide. While not wishing to be bound by any theory, it is believedthat the zinc increases the rate at which iron fluxes with the coal ash.“Ash” refers to the residue remaining after complete combustion of thecoal particles. Ash typically includes mineral matter (silica, alumina,iron oxide, etc.) Zinc is believed to act as a mineralizer. Mineralizersare substances that reduce the temperature at which a material sintersby forming solid solutions. This is especially important where, as here,the coal/ash residence time in the combustor is extremely short(typically less than about one second). Preferably, the additiveincludes at least about 1 wt. % (dry basis) mineralizer and morepreferably, the additive includes from about 3 to about 5 wt. % (drybasis) mineralizer. Mineralizers other than zinc oxides include calcium,magnesium or manganese flourides or sulfites and other compounds knownto those in the art of cement-making. Preferably, the additive includesno more than about 0.5 wt. % (dry basis) sulfur, more preferablyincludes no more than about 0.1 wt. % (dry basis) sulfur, and even morepreferably is at least substantially free of sulfur.

[0022] The injection rate of the iron-containing additive to the chamberdepends, of course, on the combustion conditions and the chemicalcomposition of the coal feed and additive. Typically, the injection rateof the iron-containing additive into the combustion chamber ranges fromabout 10 to about 50 lb/ton coal and more typically from about 10 toabout 20 lb/ton coal.

[0023] After combination with the additive, the coal-containing feedmaterial typically includes:

[0024] (a) coal; and

[0025] (b) an additive that includes iron in an amount of at least about0.5 wt. % (dry basis) and mineralizer in an amount of at least about0.005 wt.% (dry basis).

[0026] The methods and additives of the present invention can have anumber of advantages compared to conventional systems. The additive(s)can provide a slag layer in the furnace having the desired viscosity andthickness at a lower operation temperature. As a result, there is morebottom ash to sell, a relatively low flyash carbon content, moreeffective combustion of the coal, more reliable slag tapping, improvedboiler heat transfer, and a relatively low amount of entrainedparticulates in the offgas from combustion, leading to little or nodegradation in performance of particulate collectors (due to theincreased particulate load). The boiler can operate at lower power loads(e.g., 60 MW without the additive and only 35 MW with the additive asset forth below) without freezing the slag tap and risking boilershutdown. The operation of the boiler at a lower load (and moreefficient units can operate at higher load) when the price ofelectricity is below the marginal cost of generating electricity, cansave on fuel costs. The additive can reduce the occurrence of flames inthe main furnace, lower furnace exit temperatures (or steamtemperatures), and decrease the incidence of convective pass foulingcompared to existing systems. The additive can have little, if any,sulfur, thereby not adversely impacting sulfur dioxide emissions. Theseand other advantages will become evident from the following discussion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a cutaway view of a combustor of a cyclone furnace;

[0028]FIG. 2 is a graph of ash Fe₂O₃:(CaO+MgO) content ratio versustotal sulfur in coal (wt percent—dry basis) showing coals suitable for acyclone furnace;

[0029]FIG. 3 is a graph plotting slag viscosity versus operatingtemperature for high and low sulfur coals;

[0030]FIG. 4 is a first embodiment of a flow schematic of a processusing an additive according to one embodiment of the present invention;

[0031]FIG. 5 is a second embodiment of a flow schematic of a processusing an additive according to one embodiment of the present invention;

[0032]FIG. 6 is a chart of load (vertical axis) versus additive/noadditive conditions (horizontal axis);

[0033]FIG. 7 is a plot of viscosity (Cp) (vertical axis) versustemperature (horizontal axis) for various experiments; and

[0034]FIG. 8 is a plot of viscosity (Cp) (vertical axis) versustemperature (horizontal axis).

DETAILED DESCRIPTION

[0035] The Additive

[0036] As noted, the additive contains iron and preferably amineralizing agent, such as zinc. The iron and mineralizing agent can bein any form, such as an oxide or sulfide, so long as the iron andmineralizing agent will be reactive under the operating conditions ofthe furnace. Preferably, the additive includes at least about 50 wt. %(dry basis) iron and more preferably at least about 80 wt. % (dry basis)iron and even more preferably from about 70 to about 90 wt. % (drybasis) iron. Preferably, the ratio of ferric (or higher valence) iron toferrous (or lower valence) iron is less than 2:1 and even morepreferably ranges from about 0.1:1 to about 1.9:1, or more preferably atleast about 33.5% and even more preferably at least about 35% and evenmore preferably at least about 40% of the iron in the additive is in theform of ferrous (or lower valence ) iron and no more than about 65% ofthe iron in the additive is in the form of ferric (or higher valence)iron. In a particularly preferred formulation, at least about 10%, morepreferably at least about 15% of the iron is in the form of wustite, andeven more preferably from about 15 to about 50% of the iron is in theform of wustite. Preferably, the additive includes at least about 0.1wt. % (dry basis) mineralizing agent and more preferably from about 0.5to about 15 wt. % (dry basis) mineralizing agent, even more preferablyfrom about 2 to about 8 wt. % (dry basis), and even more preferably fromabout 3 to about 5 wt. % (dry basis) mineralizing agent. Due to theformation of sulfur oxides, the additive typically includes little, ifany, sulfur.

[0037] The additive is preferably in the form of a free-flowingparticulate and has a relatively fine particle size. Preferably, the P₉₀size of the additive is no more than about 300 microns, more preferablyno more than about 150 microns, and even more preferably no more thanabout 75 microns.

[0038] The additive can be manufactured by a number of processes. Forexample, the additive can be the particles removed by particulatecollection systems (e.g., by electrostatic precipitators or baghouses)from offgases of steel or iron manufacturing. Preferably, the additiveis the collected fines (flue dust and/or electrostatic precipitatordust) from the offgas(es) of a blast furnace or BOF. In such materials,the iron and mineralizer are typically present as oxides. The additivecan also be a sludge containing iron plus oils and greases producedduring metal finishing operations. Oils and greases have the advantagesof preventing fugitive emissions during handling and shipping andreplacing the heat input requirement from the coal in the boiler andthus reduce fuel costs for producing electricity. Typically, suchadditives will contain from about 0.1 to about 10 wt. % (dry basis)greases and oils. Another source of iron-containing material is red mudfrom the bauxite mining industry.

[0039] Transportation of the Additive

[0040] Because of the small size of much of the available byproductmaterial, handling and transportation of the material can result in highfugitive dust emissions. It is therefore desirable to treat the materialto provide acceptable dusting characteristics. The treatment can takeplace at the source of the material, at a transportation terminal, or atthe plant site. There are several different types of treatmentincluding:

[0041] (i) Adding water, typically in a ratio of from about 100:1 toabout 1000:1 parts material to part water, to the material. Adding waterto the material forms a cohesive layer on the wetted surface afterdrying of the material, which will substantially eliminate fugitiveemissions from the pile.

[0042] (ii) The hydrophilic nature of the iron materials also means thatthey can be mixed as a slurry and made into any form desirable forshipping. Briquettes of the material can be made to decrease dustemissions during handling.

[0043] (iii) Organic and/or inorganic adhesives can be added to theslurried material to increase the cohesiveness of the final material.Typically, such adhesives are added in the ratio of about 100:1 to about1000:1 parts material to part adhesive. Laboratory tests have shown thatxanthan gum and phosphoric acid lead to very cohesive agents.

[0044] (iv) Spraying with conventional dust suppression chemicals suchas calcium lignosulfonate can treat the material to prevent handlingproblems. This material is commonly used to reduce coal dust emissionsand can be applied at a range of concentrations of from about 1 to about10 wt. % (dry basis) of the additive at a low cost.

[0045] Use of the Additive

[0046] The additive can be contacted with the coal feed in a number ofdifferent ways. For example, the additive can be mixed with the coalfeed at a shipping terminal, added to the coal reclaim belt, added tothe coal bunkers, and/or added to the coal feed and/or primary airstreams using an eductor to aspirate the additive.

[0047] Referring to FIG. 4, a first method for adding the additive tothe combustion process will be discussed. The additive is transportedpneumatically from a hopper 200 of a covered railcar or truck using avacuum blower 204 and transport line 208. The additive-containing gasstream passes through a filter receiver 212, which collects the additiveas a retentate. The additive drops from the filter surface into thehopper 216 via duct 220. A bin vent filter 224 prevents pressure buildup in the hopper 216 and accidental release of the additive from thehopper 216 into the ambient atmosphere. A metered valve 228 permits theadditive to flow at a desired rate (typically from about 5 to about 2000lb./min.) into a feed line 232, where the additive is combined withpressurized air (via blower 236). The additive is entrained in the airand transported through splitter 240 and to a number of coal feed pipes244 a,b. The additive/air stream is combined with the coal/air streampassing through the coal feed pipes 244 a,b to form feed mixtures forthe furnace. The feed mixtures 244 a,b are then introduced into thecombustors via coal inlet 116 (FIG. 1).

[0048] The additive can be highly cohesive and have a tendency to formdense, hard deposits in the above-noted delivery system. A flow aidand/or abrasive material can be added to the material to aid in itshandling. As used herein, a “flow aid” refers to any substance thatreduces particle-to-particle attraction or sticking, such as throughelectrostatic or mechanical means. Preferred flow aids include ethyleneglycol, “GRIND AIDS” manufactured by WR Grace Inc. The preferred amountof flow aid in the additive is at least about 1 and no more than about10 wt. % (dry basis) and more preferably at least about 1 and no morethan about 5 wt. % (dry basis). Abrasive materials can also be used toprevent deposit formation and/or life. As will be appreciated, abrasivematerials will remove deposits from the conduit walls through abrasion.Any abrasive material may be employed, with preferred materials beingsand, blasting grit, and/or boiler slag. The preferred amount ofabrasive material in the additive is at least about 2 and no more thanabout 20 wt. % (dry basis) and more preferably at least about 2 and nomore than about 10 wt. % (dry basis).

[0049] Using the additive, the slag layer in the coal-burning furnacetypically includes:

[0050] (a) at least about 5 wt. % (dry basis) coal;

[0051] (b) iron in an amount of at least about 15 wt. % (dry basis); and

[0052] (c) at least one mineralizer in an amount of at least about 1 wt.% (dry basis).

[0053] When the additive is employed, the slag layer in the combustor isin the form of a free-flowing liquid and typically has a viscosity of atleast about 250 Poise.

[0054] Due to the presence of minerals in the feed material, the slaglayer in the combustor can include other components. Examples includetypically:

[0055] (d) from about 20 to about 35 wt. % (dry basis) silica oxides orSiO₂;

[0056] (e) from about 13 to about 20 wt. % (dry basis) aluminum oxidesor Al₂O₃;

[0057] (f) from about 0 to about 2 wt. % (dry basis) titanium oxides orTiO₂;

[0058] (g) from about 18 to about 35 wt. % (dry basis) calcium oxides orCaO; and

[0059] (h) from about 3 to about 10 wt. % (dry basis) magnesium oxidesor MgO.

[0060] The solid byproduct of the coal combustion process is typicallymore saleable than the byproduct in the absence of the additive. Thesolid byproduct is typically harder than the other byproduct and has ahighly desirable composition. Typically, the byproduct includes:

[0061] (a) at least about 20 wt. % (dry basis) silica;

[0062] (b) iron in an amount of at least about 15 wt. % (dry basis);

[0063] (c) mineralizer in an amount of at least about 1 wt. % (drybasis); and

[0064] (d) at least about 13 wt % (dry basis) aluminum.

[0065] The byproduct can further include one or more of the compoundsnoted above.

[0066] A second embodiment of a method for adding the additive to thecombustion process is depicted in FIG. 5. Like reference numbers referto the same components in FIG. 4. The process of FIG. 5 differs from theprocess of FIG. 4 in a number of respects. First, a controller 300controls the feed rate of the additive from the hopper 304 to thetransport conduit 308 and various other unit operations via controllines 321 a-e. For additive feed rate, the controller 300 can use feedforward and/or feedback control. The feed forward control would be basedupon the chemical analysis of the coal being fed from to the furnace.Typically, the chemical analysis would be based on the iron and/or ashcontent of the coal feed. Feedback control could come from a variety ofmeasured characteristics of boiler operation and downstream componentssuch as: LOI (flue gas 0₂ and CO with a higher O₂ and/or COconcentration indicating less efficient combustion) as measured by anon-line furnace analyzer (not shown), carbon content in ash asdetermined from ash samples extracted from the flue gas or particlecollector (e.g., electrostatic precipitator hopper) (the carbon contentis indirectly proportional to combustion efficiency), furnace exit gastemperature (which will decrease with less coal carryover from thecyclones, slag optical characteristics such as emissivity or surfacetemperature (the above noted additive will desirably reduce emissivityand increase boiler heat transfer), slag tap flow monitoring to assureboiler operability, and stack opacity (a higher stack opacity equates toa less efficient combustion and vice versa). The controller 300 furthermonitors other boiler performance parameters (e.g., steam temperatureand pressure, NO₂ emissions, et al., through linkage to a boiler digitalcontrol system or DCS. In the event of system malfunction (as determinedby a measured parameter falling below or exceeding predeterminedthreshholds in a look-up table), the controller 300 can forward an alarmsignal to the control room and/or automatically shut down one or moreunit operations.

[0067] The additive is removed from the railcar 200 via flexible hoses316 a,b with camlock fittings 320 a,b using a pressured airstreamproduced by pressure blower 324. The pressurized airstream entrains theadditive in the railcar and transports the additive via conduit 328 tothe surge hopper 304 and introduced into the hopper in an input port 332located in a mid-section of the hopper 304.

[0068] Compressed air 336 is introduced into a lower section of thehopper 304 via a plurality of air nozzles 340 a-f. The additive bed (notshown) in the hopper 304 is therefore fluidized and maintained in astate of suspension to prevent the additive from forming a cohesivedeposit in the hopper. The bed is therefore fluidized during injectionof the additive into the coal feed lines 344 a,b.

[0069] The compressed air 336 can be used to periodically clean thehopper 304 and filter 348 by opening valves 352, 356, and 360 andclosing valves 362 and 364.

[0070] Filters 366 a,b are located at the inlet of the blowers 376 and380 to remove entrained material. Mufflers 368 a,b and 372 a,b arelocated at the inlet and outlet of the blowers 376 and 380 for noisesuppression.

[0071] Finally, a number of abbreviations in FIG. 5 will be explained.“M” refers to the blower motors and an on/off switch to the motors,“PSH” to an in-line pressure sensor that transmits digital informationto the controller 300, “PI” to a visual in-line pressure gauge, “dPS” toa differential pressure switch which transmits a digital signal to thecontroller indicating the pressure drop across filter receiver 212(which compares the digital signal to a predetermined maximum desiredpressure drop to determine when the filter receiver 212 requirescleaning), “dPI” to a visual differential pressure gauge measuring thepressure drop across the filter receiver 212, “LAH” to an upper leveldetector that senses when the additive is at a certain (upper) level inthe hopper and transmits an alarm signal to the controller 300, “LAL” toa lower level detector that senses when the additive is at a certain(lower) level in the hopper and transmits an alarm signal to thecontroller 300, and “SV” to a solenoid valve that is actuated by anelectrical signal from the controller 300.

EXPERIMENTAL

[0072] The slag viscosity of a cyclone furnace was modeled and used tocompare the effects of the additive without the additive. The elementalanalysis of BOF flue dust was used as the additive. The slag viscositymodel showed that the BOF flue dust, when added to the coal to increasethe ash iron percentage to 30% by weight (dry basis), increased thethickness of the slag layer in the cyclone by about 60%.

[0073] The coal used in the model was based on the specifications forwestern coal, which is as follows:

[0074] Total ash=about 2-15% (dry basis) of the coal

[0075] SiO2=about 20-35% (dry basis) of the ash

[0076] Al203=about 13-20% (dry basis) of the ash

[0077] Ti02=about 0-2% (dry basis) of the ash

[0078] Fe203=about 3-10% (dry basis) of the ash

[0079] CaO=about 18-35% (dry basis) of the ash

[0080] MgO=about 3-10% (dry basis) of the ash

[0081] Na20=about 0-3% (dry basis) of the ash

[0082] K20=about 0-1% (dry basis) of the ash

[0083] SO3/other=about 6-20% (dry basis) of the ash

[0084] The model also showed that the temperature at which the ash wouldhave a viscosity of 250 poise would be reduced by at least 100° F. Thetemperature is an important indicator of the minimum temperature atwhich the slag will flow. If the temperature at which the ash has aviscosity of 250 poise or lower is too high, then the slag will not flowto the slag tap on the floor of the boiler, and the slag will build upinside the boiler casing. This has been a problem on cyclone furnacesburning western coal at less than full design output.

[0085] The first field test of the additive took place at a 75 MW unitin the midwest. A pneumatic storage and injection system was installedat the site, and boiler performance data was obtained during April of2000. The changes in boiler operation were dramatic as shown in FIG. 6.In FIG. 6, “ADA-249” refers to the additive of the present invention.

[0086] Based on FIG. 6 and other experimental information, variousobservations may be made regarding the performance of ADA-249.

[0087] Minimum load was reduced from 75% to 47% of rated capacity whenusing only about 20 lb. of the additive per ton of coal.

[0088] The cost impact on load dispatch was about $200 K/y, not countingthe expected increase in unit availability from fewer shutdowns to cleanthe “monkey hole”.

[0089] A high-temperature video camera also showed that the main furnaceis clear when injecting the additive (meaning that the coal stays in thecyclone to burn) instead of hazy due to unburned fuel when no additiveis injected.

[0090] The plant confirms that flyash LOI is low and bottom ash isacceptable for high-value sale when the additive is on.

[0091] While all iron compounds will flux and thicken the slag layerwhen burning low-sulfur coals, the effects are improved by incorporatinga blend of reduced iron compounds such as Wustite (FeO) and Magnetite(Fe₃O₄). FIG. 7 shows this effect. This figure shows temperature andviscosity data for a typical slag alone (shown as “No Additive”),compared to the same slag treated with 9 wt. % (of the slag (dry basis))magnetite or 12 wt. % (of the slag (dry basis)) wustite at levels togive the same percent iron in the mixture. It can be seen that wustiteallows slag flow at a lower temperature. Further, wustite contributesiron crystals to the melt (as indicated by the sharp rise in the curve)at a lower temperature. Wustite is comparatively rare in nature, but isa byproduct of the BOF processes.

[0092] The present invention can also be applied to eastern low-sulfurcoals having very high ash melting temperatures. FIG. 8 compares theviscosity-temperature relationships of coal slag alone (shown as“Coffeen (rd.)”), against the same coal slag treated with 2 percentlimestone (shown as “Coffeen+limestone (rd.)”) or 2 percent of theadditive (shown as “Coffeen+ADA-249 (rd.)”). The horizontal line 400denotes the value of 250 poise. The basis for this comparison is theT₂₅₀, a slag characteristic used by fuel buyers to select the propercoal for cyclone furnaces. This value represents the temperature belowwhich the slag will not flow out of the cyclone combustor.

[0093] The slag without additive has a T₂₅₀ of about 2500° F., which isslightly higher than the maximum recommended T₂₅₀ of 2450° F. By adding2% limestone, the T₂₅₀ can be lowered into the acceptable range (around2200° F.). However, the same amount of the additive was able to reducethe T₂₅₀ to below 1900° F. Looking at it another way, the T₂₅₀ coalrequirement could be satisfied by adding half as much of the additive aslimestone. Because of the increased effectiveness of the additive of thepresent invention, it becomes an economic alternative to limestone foreastern bituminous coals.

[0094] While various embodiments of the present invention have beendescribed in detail, it is apparent that further modifications andadaptations of the invention will occur to those skilled in the art.However, it is to be expressly understood that such modifications andadaptations are within the spirit and scope of the present invention.

What is claimed is:
 1. A method for combusting coal, comprising:providing a coal-containing feed material to a coal combustion chamber;contacting the feed material with an iron-containing additive, whereinthe iron-containing additive is in the form of a free-flowingparticulate having a P₉₀ size of no more than about 300 microns; andmelting at least a portion of the coal-containing feed material andiron-containing additive to form a slag layer on at least a portion of asurface of the combustion chamber, whereby coal in the coal-containingfeed material is captured by the slag layer and combusted.
 2. The methodof claim 1, wherein the coal-containing feed material comprises ash andless than about 1 wt. % (dry basis of the coal-containing feed material)sulfur, with the ash containing less than about 10 wt. % (dry basis ofthe ash) iron, and at least about 20 wt. % (dry basis of the ash)alkali.
 3. The method of claim 1, wherein the additive further compriseswustite.
 4. The method of claim 1, wherein the coal combustion chamberis part of a cyclone furnace and wherein the coal-containing feedmaterial includes coal particles entrained in an oxygen-containing gasand the coal particles have a P₉₀ size of no more than about 0.25inches.
 5. The method of claim 1, wherein the iron-containing additivecomprises at least about 50 wt. % (dry basis of the additive) iron andat least about 1 wt. % (dry basis of the additive) mineralizer.
 6. Themethod of claim 5, wherein the mineralizer is zinc and theiron-containing additive further comprises at least one of a flow aidand abrasive material.
 7. The method of claim 1, wherein an injectionrate of the iron-containing additive into the combustion chamber rangesfrom about 10 to about 50 lb/ton coal.
 8. A method for combusting coal,comprising: inputting a coal-containing feed material to a coalcombustion chamber; contacting the feed material with an iron-containingadditive, wherein the iron-containing additive comprises at least about50 wt. % (dry basis of the additive) iron and at least about 1 wt. %(dry basis of the additive) mineralizer; and melting at least a portionof the coal-containing feed material and iron-containing additive toform a slag layer on at least a portion of a surface of the combustionchamber, whereby coal in the coal-containing feed material is capturedby the slag layer and combusted.
 9. The method of claim 8, wherein thecoal-containing feed material comprises ash and less than about 1.5 wt.% (dry basis of the coal-containing feed material) sulfur, with the ashcontaining less than about 10 wt. % (dry basis of the ash) iron, and atleast about 15 wt. % (dry basis of the ash) calcium.
 10. The method ofclaim 8, wherein the mineralizer is a zinc compound and thecoal-containing feed material includes coal particles entrained in anoxygen-containing gas and the coal particles have a P₉₀ size of no morethan about 0.25 inch.
 11. The method of claim 8, wherein the coalcombustion chamber is part of a cyclone furnace.
 12. The method of claim8, wherein the iron-containing additive is in the form of a free-flowingparticulate having a P₉₀ size of no more than about 300 microns.
 13. Themethod of claim 8, wherein the iron-containing additive is a byproductof steel or iron manufacturer and comprises wustite.
 14. The method ofclaim 8, wherein an injection rate of the iron-containing additive intothe combustion chamber ranges from about 10 to about 50 lb/ton coal andthe iron-containing additive further comprises at least one of a flowaid and an abrasive material.
 15. A method for combusting coal,comprising: introducing a coal-containing feed material to a coalcombustion chamber; contacting the feed material with an iron-containingadditive, wherein the iron-containing additive comprises at least about50 wt. % (dry basis of the additive) iron and the iron is in the form ofa mixture of ferrous and ferric oxides; and melting at least a portionof the coal-containing feed material and iron-containing additive toform a slag layer on at least a portion of a surface of the combustionchamber, whereby coal in the coal-containing feed material is capturedby the slag layer and combusted.
 16. The method of claim 15, wherein theadditive comprises at least about 1 wt. % (dry basis of the additive)mineralizer.
 17. The method of claim 15, wherein the mineralizer is azinc compound, wherein the coal-containing feed material includes coalparticles entrained in an oxygen-containing gas, wherein the coalparticles have a P₉₀ size of no more than about 0.25 inch, and whereinthe coal-containing feed material comprises ash and less than about 1.5wt. % (dry basis of the coal-containing feed material) sulfur, with theash containing less than about 10 wt. % (dry basis of the ash) iron, andat least about 15 wt. % (dry basis of the ash) calcium.
 18. The methodof claim 15, wherein the coal combustion chamber is part of a cyclonefurnace and at least some of the iron is in the form of wustite.
 19. Themethod of claim 15, wherein the iron-containing additive is in the formof a free-flowing particulate having a P₉₀ size of no more than about300 microns.
 20. The method of claim 15, wherein the iron-containingadditive is a byproduct of steel or iron manufacturer and compriseswustite.
 21. An additive for a coal feed to a cyclone furnace,comprising: iron in an amount of at least about 50 wt. % (dry basis ofthe additive), wherein the iron comprises at least about 33.5% of theiron is in the form of iron having a valence of +2 or lower and whereinthe additive is in the form of a free-flowing particulate having a P₉₀size of no more than about 300 microns.
 22. The additive of claim 21,wherein the iron content ranges from about 50 to about 99 wt. % (drybasis of the additive) and further comprising a mineralizer in an amountof at least about 1 wt. % (dry basis of the additive).
 23. The additiveof claim 22, wherein the mineralizer is zinc and the zinc content rangesfrom about 1 to about 8 wt. % (dry basis of the additive).
 24. Theadditive of claim 21, further comprising at least one of: (a) a flow aidin an amount of at least about 0.1 wt. % (dry basis of the additive) and(b) an abrasive material in an amount of at least about 1 wt. % (drybasis of the additive).
 25. A coal-containing feed material for afurnace, comprising: coal; and an additive, wherein the additiveincludes: iron in an amount of at least about 50 wt. % (dry basis of theadditive) and a mineralizer in an amount of at least about 1 wt. % (drybasis of the additive).
 26. The material of claim 25, wherein theadditive is in the form of a free-flowing particulate having a P₉₀ sizeof no more than about 300 microns.
 27. The material of claim 25, whereinthe iron content of the additive ranges from about 50 to about 99 wt. %(dry basis of the additive), wherein the coal and additive are entrainedin an airstream, and wherein the additive includes wustite.
 28. Thematerial of claim 25, wherein the mineralizer content of the additiveranges from about 1 to about 8 wt. % (dry basis of the additive). 29.The material of claim 25, wherein the material comprises at least about15 wt. % (dry basis of the coal) alkali and no more than about 1.5 wt. %(dry basis of the coal) sulfur.
 30. The material of claim 25, whereinthe coal includes from about 3 to about 20 wt. % ash (dry basis of thecoal).
 31. The material of claim 25, further comprising at least one ofa flow aid and an abrasive material.
 32. A slag layer in a coal-burningfurnace, comprising: at least about 5 wt. % (dry basis of the slaglayer) coal; iron in an amount of at least about 15 wt. % (dry basis ofthe slag layer); and a mineralizer in an amount of at least about 1 wt.% (dry basis of the slag layer).
 33. The slag layer of claim 32, whereinthe slag layer is in the form of a free-flowing liquid having aviscosity of no more than about 250 Poise.
 34. The slag layer of claim32, wherein the iron content ranges from about 15 to about 30 wt. % (drybasis of the slag layer).
 35. The slag layer of claim 32, wherein themineralizer is zinc and the zinc content ranges from about 1 to about 8wt. % (dry basis of the slag layer).
 36. The slag layer of claim 32,wherein the slag layer comprises at least about 15 wt. % calcium and nomore than about 1 wt. % sulfur (dry basis of the slag layer).
 37. Theslag layer of claim 32, further comprising at least about 20 wt. %silica oxides (dry basis of the slag layer).
 38. The slag layer of claim37, wherein and further comprising from about 13 to about 20 wt. %aluminum oxides (dry basis of the slag layer); from about 18 to about 35wt. % calcium oxides (dry basis of the slag layer).
 39. A solidbyproduct of coal combustion, comprising: iron in an amount of at leastabout 15 wt. % (dry basis of the byproduct); zinc in an amount of atleast about 1 wt. % (dry basis of the byproduct); silica compounds in anamount of at least about 2 wt. % (dry basis of the byproduct); aluminumcompounds in an amount of at least about 20 wt. % (dry basis of thebyproduct); and calcium compounds in an amount of at least about 15 wt.% (dry basis of the byproduct).
 40. The solid byproduct of claim 39,wherein the iron content ranges from about 15 to about 30 wt. % (drybasis of the byproduct).